This invention relates generally to the vapor deposition of thin films and more particularly to a method of changing or modifying properties of the growth of semiconductor materials during epitaxial growth via chemical vapor deposition (CVD) and more specifically to a method of making in situ stoichiometric (e.g., atomic molar fraction changes) and geometrical (e.g., layer thickness changes) modifications to binary, ternary and other compound semiconductor thin films, such as, II-VI or III-V compounds (e.g., GaAs) or alloys (e.g. GaAlAs), during their epitaxial growth (e.g., metalorganic vapor phase epitaxy or metalorganic chemical vapor deposition, i.e., (MOVPE or MOCVD).
Much work is being accomplished at this point in time relative to the use of photo assistance during the CVD-of thin films of materials. Such processing has been referred to as laser CVD or LCVD, or laser assisted or induced CVD. A recent publication summarizes much of this work and refers to it as "laser microchemical processing": F. Micheli and I. W. Boyd, "Laser Microfabrication of Thin Films: Part One, Part Two and Part Three", Optics and Laser Technology, Part One:Vol. 18(6), pp. 313-317, December 1986; Part Two: Vol. 19(1), pp. 19-25, February, 1987; and Part Three: Vol. 19(2), pp. 75-82, April, 1987. References in the patent literature include methods of selective depositing primarily via pyrolysis, e.g., U.S. Pat. No. 4,543,270; via photolysis, e.g., U.S. Pat. Nos. 4,608,117; 4,668,528; 4,678,536; 4,693,779 and 4,726,320; or via a combination of photolysis and pyrolysis, e.g., U.S. Pat. Nos. 4,579,750 and 4,581,248.
Laser assisted molecular beam epitaxy (MBE) has been proposed and developed as exemplified in U.S. Pat. No. 4,071,383 to Nagata et al. Nagata et al. discloses selective stoichiometric changes in an epitaxially deposited film in MBE wherein higher refractive index material is produced in beam irradiated areas of the depositing film as compared to unirradiated areas producing an optical embedded waveguide. Laser assisted metalorganic chemical vapor deposition (MOCVD) is now being developed as a versatile means of patterning the growth of III-V compounds. In a typical laser assisted MOCVD process, an in situ laser beam is irradiated onto a portion of a substrate during growth. Depending upon the optical intensity of the beam and substrate temperature, the laser radiation photochemically and/or photothermally increases the crystal growth rate. In this manner, selective growth of GaAs has been demonstrated with a wide range of photon energies (2.4-6.4 eV) and optical intensities. See, for example, the articles of W. Roth et al,. laser stimulated growth of Epitaxial GaAs Material Resource Society Symposium Proceeding, entitled "Laser Diagnostics and Photochemical Processing for Semiconductor Devices," Vol. 17, pp. 193-198, 1983; Y. Aoyagi et al, Applied Physics Letters, Vol. 47(2), pp. 95-96, Jul. 15, 1985; S. M. Bedair et al, Applied Physics Letters, Vol. 48(2), pp. 174-176, Jan. 13, 1986, H. Kukimoto et al, Journal of Crystal Growth, Vol. 77(1-3), pp. 223-228, Sep. 1986; and T. Soga et al, Journal of Crystal Growth, Vol. 68(1), pp. 169-175, September, 1984.
Selective growth of the ternary compounds GaAsP (.lambda.=514.5 nm) and GaAlAs (.lambda.=248 nm) via laser assisted MOCVD is respectively demonstrated in S. M. Bedair et al and H. Kukimoto et al. Not only is the growth rate increased for ternary compounds, but also the stoichiometry is affected by the laser radiation. For example, in Kukimoto et al, a slight increase in Al incorporation has been shown to be induced with excimer radiation (193 nm) during the epitaxial growth of GaAlAs. The Al content in this ternary was shown to increase in irradiated areas compared to unirradiated areas of the depositing film. Also, the Al content incorporation increased slightly with temperatures in the range of 600.degree. C. to just over 700.degree. C. These stoichiometric changes in the growth of GaAlAs occurred at temperatures at about 600.degree. C. and below and these stoichiometric changes occurred for different transport rate ratios of the deposition gases involved. It was further observed by Kukimoto et al that the growth ratios for GaAs and GaAlAs layers were not influenced by laser irradiation at growth temperatures higher than 600.degree. C. Kukimoto et al suggests that selective area control of material properties, i.e., the selective control of Al content in the growth of GaAlAs, has potential for fabrication of various semiconductor devices because the selective differences in Al molar fraction during growth brings local differences in optical properties, such as refractive index and in electrical properties, such as energy bandgap of the material. Since laser assisted MOCVD enhances the incorporation of Al during the growth of GaAlAs, such laser assistance processing may be used to locally vary the bandgap of a GaAlAs layer or thin film by controlling the content of Al incorporated into the thin film via optical illumination applied in situ during epitaxial growth.
The recently issued patent to Maslov et al, U.S. Pat. No. 4,117,504, is an example of another method for bringing about stoichiometric change during epitaxial growth but does not involve photo assisted CVD. Maslov et al discloses apparatus for the solid state evaporation of sequentially aligned semiconductor compound materials as the same are transversely passed in opposed relation to a heated substrate. As a result, a thin film is deposited on the substrate having a monotonically increasing stoichiometric change in deposited material laterally across the substrate surface. This change is referred to as a composition gradient and an example in the patent disclosure is a stoichiometric change across a deposited thin film from GaAs to GaAs.sub.0.64 P.sub.0.36.
It is an object of this invention to bring about in situ stoichiometric and growth rate changes in compound semiconductors employing photo assisted MOCVD epitaxy and to apply this method of photo assisted MOCVD epitaxy in the fabrication of devices, such as multiple wavelength light emitting LED'S, semiconductor lasers or laser arrays.